fix: make gradientWrapper fill parent

[alexz88dev]

Summary

This PR updates the Shimmer component's internal gradientWrapper to use StyleSheet.absoluteFillObject.

Previously, the animated gradient had a fixed internal width of 100px. This change makes the gradient dynamically fill the entire area of the Shimmer component, providing a more intuitive full-area shimmer effect by default. Gradient customization props (linearGradients, gradientStart, gradientEnd) will now apply relative to the component's full dimensions.

Test plan

1. Render a Shimmer component.
   • Expected: The shimmer animation now covers the entire component area, not just a 100px wide band.
2. Render a Shimmer component wider than 100px with custom linearGradients.
   • Expected: The custom gradient colors now span the full width of the component.